Inflammatory-caspases critical for apoptosis in Sjögren's syndrome prior to disease-onset

Objectives: Sjögren's syndrome (SjS) is a chronic autoimmune disease targeting the salivary and lacrimal glands. Studies from microarray analyses indicated potential involvement of inflammatory caspases (caspases-11 and -1) in the submandibular glands (SMX) of SjS-prone-C57BL/6.NOD-Aec1Aec2 mice prior to disease onset. To determine if inflammatory caspases are critical for the increased epithelial cell death prior to SjS-like disease in the mouse model, we investigated molecular events involving caspase-11/caspase-1 axis in vivo as well as in vitro.

Methods: RT-PCR, EMSA, ELISA, caspase activity assay, TUNEL staining, immunohistochemistry and small interfering RNA (siRNA) gene inhibition experiments were performed.

Results: Our data revealed concurrent up-regulation of caspase-11 in salivary macrophages, STAT-1 activity, caspase-1 activity, and apoptotic epithelial cells in the SMX of C57BL/6.NOD-Aec1Aec2 during pre-disease stage. Caspase-1, a critical factor for IL-1beta and IL-18 secretion, resulted in elevated level of IL-18 in saliva. Interestingly, TUNEL-positive cells in the SMX of C57BL/6.NOD-Aec1Aec2 were not co-localized with caspase-11, indicating that caspase-11 functions in a non-cell autonomous manner. To investigate if activated caspase-11/caspase-1 pathway is associated with increased apoptosis, human acute monocytic leukemia cells (THP-1) were co-cultured with human salivary gland (HSG) cells in the presence and absence of LPS (1µg/ml) and/or IFN-gamma (10 ng/ml) for 24 and 48 hours. Interestingly, increased apoptosis of HSG cells occurred only in the presence of LPS-and IFN-gamma-stimulated THP-1 cells, which was reversed when caspase-1 in THP-1 cells was targeted by siRNA.

Conclusion: Taken together, our study discovered that inflammatory caspases in macrophages are essential in promoting pro-inflammatory microenvironment and subsequent epithelial cell death in the target tissues of SjS before disease onset. Synergistic effects of the cytokines appear to be necessary for this process. If target tissue apoptosis is required and/or necessary in the onset of SjS still needs to be further investigated (supported by NIH/NIDCR grants U24DE016509 & R21DE16705).